Posted
by
Cliffon Wednesday February 04, 2004 @05:30PM
from the tracking-the-coverage dept.

NivekEnterprises asks: "At my school where I am an Electrical Engineering major A Wi-Fi network has been set up, and is running in several buildings. Since the coverage extends outside several of my friends and I are trying to map the signal strength. Basically we are going to take a map of campus and walk around with a Wi-Fi enabled laptop, marking as best we can. Is there an easier way to do this? Has anyone else done something similar and is willing to share some their insights?"

Netstumbler software allows the existing laptop (the kids seem to have that already) to act as the detector. I know it measures signal strength as well as can interface with a GPS, but I do not know if it will map signal strength to position, or if it is strictly binary.

If you want to do this right I suggest you use kismet, a GPS receiver hooked up to the laptop, and use the most common configuration of wifi card. Don't use anything to boost the signal, assume that if someone can use an antena, they don't need you maps. Or make two sets of maps. Then set out with the data kismet collected and make a map. It's really easy to do this. GPS drive can help you with parts of it but it's not going to be entirely effective.

There are plenty of GPS devices out there you can plug into your PCMCIA slots (or maybe even via USB). They probably will cost you about 100-200 US. After that, you can run around your campus and "war" it out. You will be able to see where you walked on your laptops map (though the buildings of your school will not be written unless you have a map of it already built in).

The program will show you what spots are hot, what is not...
Good luck!

Why are you making this map?
Anyway walking around with your laptop computer may be the cheapest way to get your map. You can always just ask the school for where they placed all their wi-fi equipment. It should be public information and you can largely base your maps on that.

Get yourself a GPS usb device. You can collect raw data from it and easily overlay the signal strength to a school map. You might run into trouble though if the map isn't scaled properly though.
This way you can just walk around and have the laptop do everything.

The first is to create a link budget using the
two-ray approximation [earthlink.net] (1/R^4 attenuation)
and the estimated antenna patterns.
This allows one to bound the maximum range as
a function of antenna orientation and receiver sensitivity.
Most likely this is the analysis that was done
when installing the network.

A second and much more enjoyable way is to use a
ray-trace simulation program such as
Wireless InSite [remcominc.com]
to model your campus.
This model will pick up multipath effects and
folliage losses.

The most time consuming but most accurate
method is to walk around with an antenna,
measuring the power as one goes.
It should be noted that when one measures
in a given location the power will change
over time sometimes quickly.
Known as Rayleigh fading, it is due to
time-varying multipath from a dynamic environment.

I recommend tracking down a communications professor
in EE and borowing their copy of Wireless InSite. If you pitch
it right you could even get a credit of independent study
from it.

If you use Linux on your laptop, kismet [kismetwireless.net] will interface with GPS devices, and do signal power interpolation to find signal sources. It will also mark everything on a user-supplied map. Good Luck.

www.kismetwireless.com
search sourceforge for gpsd.
Hook your GPS to your linux machine and get gpsd running. Configure kismet (sniffer) to use gpsd. Kismet will record the gps location, SSID, and strength in a log file. There is even another tool included to automatically generate maps for you.
--Chris

Forget the map. What you need to do is find a way to get an analog signal strength reading (either with a device designed for the job, or hacking something on a laptop to (say) pipe a running digital signal strength value out the dsp to the earphone jack) and run it into an op-amp setup that will 1) give you decent current and 2) give you an inverted signal as well. Run these two signals to two actuators. Hook the inverted one to a can of red spray paint,and the un-inverted one to a can of blue spray pant.

Do this with the power off.

Go outside, make sure the cans are facing away from you, power up, and walk around.

From then on, the parts of campus with a blue tinge will be the ones with good signal, and the red areas will be the ones to avoid. Easy rule to learn, and no maps to deal with.

-- MarkusQ

P.S. You may need to go over some parts of campus (esp. plants, artwork, etc. that may be strongly coloured) with a coat of gray primer first. I suggest you just play around with it till you get something usable.

We tried a few different approaches. It turned out that the fastest approach was to send someone around with wireless card tools that showed SNR (signal to noise ration). In offices, the SNR at the most likely use location (e.g. the desk) for the "best" AP (access point) was measured. In classrooms, they broke big rooms into smaller chunks, then found the best SNR for each chunk.

An approach using GPS was tried, but it took longer to wait for the GPS to stabalize

Build a small blimp with a payload consisting of an embedded linux SBC with GPS and WiFi recievers. Program it to fly along arbitrary signal strength boundaries (think lines of flux) and chart its location either to an onboard storage location, like CF, or broadcast it back to a server on the WiFi network, then layer the GPS info onto your campus map like its GIS.

Who want's to go out doors and walk around exposed under that burning ball of gas? Really?
Well, you can still do that, but here's what I would do before going outdoors, and this may even provide the level of accuracy you need.

First, make a digital map of your school. As simple as scanning/cleaning up a paper map given to new students, or snag one from your local city hall's zoning department (of course, doing that may end you up on an FBI watch list).
If you really want to geek out, grab a friend and hav

GPS is the best way to go. Failing that, I used a system a bit like the following:Every ~25 seconds, laptop records WiFi signal strengths and network names and such to record #N, and emits a pair of tones corresponding to the last digit of N (I used musical intervals and the first tone was always the same; for the zero digit, the second note of the pair was null---in other words, a single tone). I carried a clipboard, which I would number as I went along, and fill in my approximate location. I could make

I highly recommend KisMAC, works great, never played with the GPS bit but it autogenerates a map. Two cavets - it only works on Jaguar and, while it supports Airport Extreme, no passive mode. It does, however, support a long list of PCMCIA cards and quite a few in passive.